Environmentally friendly coating compositions, bonding solution, and coated parts

ABSTRACT

An aqueous phosphoric bonding solution and coating composition slurry which are environmentally friendly in that they are generally free of molybdate or chromium. Yet the solution is stable with respect to inorganic metal particles, especially aluminum which are admixed to the bonding solution for the preparation of the coating slurry. The metal coated parts and the method of coating the parts with the coating composition are also described. The parts have very satisfactory properties.

This is a continuation of application Ser. No. 08/364,786, filed Dec.27, 1994, now U.S. Pat. No. 5,478,413.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates compositions and methods for providingcorrosion protection to metal substrates, with bonding solutions andcoating compositions for ferrous metal alloy surfaces which are free orat least substantially free of carcinogenic or toxic metals likechromate and molybdate, and to coating compositions comprising suchbonding solutions and aluminum or like pigments.

2. Background Art

Compositions comprising phosphoric acid and aluminum metal are wellknown for use in protecting metallic surfaces such as ferrous surfacesfrom corrosion. In such coating compositions, particulate metallicaluminum, such as flake and/or powder, is combined with a phosphoricacid bonding solution to form a coating composition which is thenapplied to the metallic surface being treated. After application of thecoating to the substrate, it may be heated to a first temperature,generally upwards of 500° F. (260° C.), until the coating is renderedessentially water insoluble. Then the coated surface may be cured at asecond temperature, generally above 1000° F. (538° C.) to form the finalprotective coating.

A problem which arises in this coating process, is that when theparticulate aluminum is combined with the phosphoric acid bondingsolution, the acidic bonding solution can react with the aluminum. Suchreactions can be very violent, causing the aluminum powder to burn oreven explode, or less violent, simply resulting in the conversion of themetallic aluminum into various salts. In either case, such reactionsinterfere with the formation of suitable protective coatings.

Allen, U.S. Pat. No. 3,248,251, issued Apr. 26, 1966, describes coatingcompositions consisting essentially of a slurry of solid inorganicparticulate material (such as aluminum) in an aqueous acidic bondingsolution containing dissolved metal chromate, dichromate or molybdate,and phosphate. It was found that the addition of chromates or molybdatesto the acidic bonding solution effectively passivated the solutiontoward aluminum and inhibited the oxidation of metallic aluminum,allowing particulate aluminum to be combined with the bonding solutionwithout the undesirable chemical reaction between the acidic solutionand the aluminum. These "Allen" coatings have been and still aresuccessfully used to provide high quality coatings which protect ferrousmetal alloy surfaces from oxidation and corrosion, particularly at hightemperatures.

However, while chromates and molybdates have been used successfully toreduce the reactivity of the aluminum in such coating compositions, theuse of chromates and molybdates has become a problem because ofenvironmental considerations. Chromates and molybdates are generallyconsidered to be toxic substances. Hexavalent chromium is rated as acarcinogen. Molybdenum is classified as a toxic heavy metal. It istherefore desirable to avoid the use of solutions of such salts, or atleast to reduce their use. For this reason, it has become desirable todevelop a phosphate/aluminum coating composition which requires littleor no chromates or molybdates to control the reactivity between theacidic phosphate bonding solution and the particulate aluminum addedthereto. The coating compositions should protect ferrous metal alloysurfaces from the oxidation and corrosive environmental conditions,especially at high temperatures, approximately as well as and preferablybetter than the so called Allen type coatings.

Recently efforts have been made to overcome the environmental problemassociated with chromates and molybdates. Stetson et al., U.S. Pat. No.5,242,488, issued Sep. 7, 1993 describes a coating composition forferrous alloys which does not require either chromates or molybdates tocontrol the reaction between the bonding solution and the powderedaluminum. The composition consists essentially of a slurry mixture of abonding solution and aluminum powder. The bonding solution consistsessentially of water, phosphoric acid (H₃ PO₄), and aluminum in anamount sufficient to substantially equilibrate the bonding solution withrespect to aluminum. The bonding solution component of the coatingcomposition of this Stetson patent requires that it contains sufficientaluminum in solution so that it is substantially equilibrated withrespect to aluminum, i.e., that the amount of aluminum in solution besubstantially at the saturation point and therefore, essentially inertwith respect to any subsequent additions of aluminum.

The Stetson patent teaches magnesium, while not essential, may desirablybe used to at least partially neutralize the aqueous phosphoric acidmixture, either before or after equilibration of the mixture withaluminum. The magnesium compound is either MgO or MgCO₃. All examplesgiven in the patent utilize magnesium ions.

An even more recent patent to Stetson, et al., U.S. Pat. No. 5,279,649,issued Jan. 18, 1994, discloses substantially the same compositions towhich V₂ O₅ has been added to produce vanadate ion, adding anotherinhibitor to the aluminum equilibrated mixture. Further, in U.S. Pat.No. 5,279,650, also issued on Jan. 18, 1994, Stetson, et al. discloses aseal coating composition of the coating disclosed in the '649 companionpatent which also contains ion oxide (Fe₂ O₃) powder. All these threecoating compositions are designed to avoid the use of the chromium andmolybdenum ions and require the bonding solution to be equilibrated withrespect to further additions of aluminum as described in these patents.The addition of V₂ O₅ demonstrates the addition of a toxic substance,listed on the OSHA extremely hazardous substance list.

Although Stetson indicates that this formulation controls the reactivitybetween the bonding solution and the aluminum, it has been discovered inwork in connection with this invention that some reaction still occursbetween the bonding solution and the powdered aluminum when the slurrycomposition of Stetson is formulated.

It is therefore desirable to formulate a chromium and molybdenum freebonding solution, or one which is of reduced chromium and molybdenumcontent, which has reduced reactivity with particulate aluminum when thetwo are combined to form a coating composition and also to be free oftoxic additives. This invention contributes to this objective.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided an improvedbonding solution for use in forming a ferrous alloy coating compositioncomprising the bonding solution and particulate metal particles likealuminum.

The present invention is also directed to a coating compositioncomprising the bonding solution of the present invention and particulatemetal particles like aluminum. The particulate aluminum may be of anysuitable form, including powder or flake, or a combination of powder andflake.

The present invention also provided a method of protecting an alloysubstrate by applying the present coating composition, and then heatingthe coated substrate to cure the coating composition and form acorrosion protected surface.

The invention also provides coated metal parts coated with the curedcoating compositions of the invention.

The bonding solution of the present invention comprises an aqueoussolution of phosphoric acid and a combination of at least one cation andone anion other than chromium and/or molybdenum to reduce the reactivityof the bonding solution to the particulate aluminum with which it may besubsequently combined to form a coating composition. Preferably, theions are zinc and borate. The bonding solution may optionally containaluminum in solution. The amount of aluminum in solution is in an amountless than necessary to reach the saturation point, thus, less the thanthe amount necessary to equilibrate the bonding solution with respect toadditional aluminum. In other words, the amount of aluminum is less thanthat which Stetson teaches is necessary to equilibrate the bondingsolution so that it will be essentially inert when powdered aluminum issubsequently admixed to the bonding solution to form the coating slurry.Although the bonding solution of the invention is not in equilibrium orsaturated, as in the prior art, the bonding solution is stable, i.e.,non-reactive with respect to particulate aluminum.

The bonding and coating compositions of the invention are preferablyfree or substantially free of chromium, especially hexavalent chromiumand molybdate and other toxic metals, like nickel or vanadium. If theycontain one or more of such metals, these may be present in amounts lessthan present in the prior art, like in the above-identified Allenpatent. The composition of the invention, though free or substantiallyfree of such objectionable metals, are stable for periods of timeadequate to apply the coating, especially for periods exceeding 1 hour,preferably more than 4 or more than 8 hours. Others are stable forseveral days and remain liquid for many weeks. The coatings are verysatisfactory, in general come up to the standards of the Allen coatingsin terms of resistance to oxidation and corrosion especially at hightemperature. The coatings of the invention are especially well suitedfor turbine compressor airfoils, like blades, vanes, stators, etc.

Although one or more individual components of the bonding solution mayhave low or reduced solubility or miscibility in water or in the aqueousphosphoric acid, ideally the bonding composition as a whole should be anaqueous solution. It is recognized, however, that some of the lesssoluble or miscible components may be present in emulsion or othernon-solution form. Thus, in accordance with the invention, the term"aqueous bonding solution" or "bonding solution" is intended to includea composition in which one or more of its components may not be fullydissolved, but may be emulsified or dispersed or in other form. Thisstatement is intended to apply to component(s) here described and othersnot described.

For best bonding results, the bonding solution should have a pH in therange of about 2.0 to about 4.5, preferably about 2.5 to about 3.0, andmore preferably about 2.7 to 3.0. To obtain the desired pH, additionalamounts of acid or base may be added to the composition. To lower the pHby the addition of acid, a water-soluble phosphoric acid or acid saltsuch as magnesium dehydrogen phosphate may be used.

The coating composition of the invention comprises the bonding solutionand a solid particulate metallic material, generally aluminum powder.Instead of or in addition to aluminum, there may be used other metalparticles, as disclosed in the above referenced Allen patent and in thepatents referenced herein below which discuss aluminum or other metalparticles, which are incorporated herein by reference. The coatingcomposition may contain additional metals in solution, like pigmentsthere may be another or additional source of borate and/or zinc, likezinc molybdate phosphate, zinc aluminum phosphate, zinc phosphate andother partially soluble (leachable) compounds, so called corrosioninhibitors. The coating composition can contain any other componentsconventionally in the coating industry. The coated ferrous parts exhibitvery satisfactory properties equivalent or better than those shown bythe Allen and Stetson patents.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, there is provided an improvedbonding solution for use in preparing a coating composition comprisingthe bonding solution and particulate metal particles like aluminum.

The bonding solution is an aqueous solution which contains phosphoricacid, one or more sources of magnesium ions and a source of zinc and ofborate ions. Another aqueous bonding solution of phosphoric acidcomprises one or more sources magnesium ions and a source of aluminumions to be dissolved in the phosphoric acid solution. Another bondingsolution contains, in addition to a source of magnesium ions, a sourceof aluminum ions and a source of either one or both zinc and borateions.

The bonding solution is preferably adjusted to a pH in the range of 2 to4.5.

The amount of aluminum ions in solution is such that it is in an amountinsufficient to substantially equilibrate the bonding solution withrespect to metal particles like aluminum, which are added in formulatingthe slurry of the coating composition. The aluminum ions dissolved inthe bonding solution is thus an amount less than that which formundissolved aluminum in the container. Such an amount of unreacted orprecipitated aluminum or aluminum compound would indicate that thereaction between the aluminum ions and the phosphoric acid has gone tocompletion and equilibrium had been reached. As discussed above, whenaluminum ions are present in the bonding solution of the invention, itis in an amount insufficient to neutralize the bonding solution withrespect to aluminum particles.

The bonding solution of the invention is stable, that is unreactive orsubstantially unreactive (or inert) to aluminum particles addedsubsequently. In accordance with a reactivity test described below, thecoating composition of the invention will show no or essentially novisible reaction when aluminum particles are admixed to the bondingsolution for at least up to one hour, preferably for up to 4 hours, morepreferably up to 8 hours and ideally for more than 8 hours.

The magnesium ions in the bonding solution may be supplied by way of anyconvenient source like in the form of magnesium carbonate, magnesiumoxide or hydroxide, etc. which will dissolve in the phosphoric acidforming the metal ions, water and/or gas which is evolved. The magnesiumcan be added magnesium. It is convenient to add magnesium oxide and/ormagnesium carbonate. The amount added alone or in combination with theother compounds will be sufficient to bring the pH within the desiredrange of 2 to 4.5 or somewhat below or above so that upon addition ofthe other compounds, e.g., the zinc and/or the borate, the pH will bewithin that range.

Other suitable magnesium compounds which can serve as a suitable sourceof magnesium ions are listed in the Handbook of Chemistry and Physics("Handbook"), 62nd Ed. CRC Press, Inc. Boca Raton, Fla., Editor Weastand Astle in the Chapter on Physical Constants of Inorganic Compounds,which is incorporated herein by reference.

The zinc ions in the bonding solution may likewise be supplied by way ofany convenient source like in the form of the metal oxide, hydroxide,carbonate, zinc phosphate, zinc moly/phosphate etc. A suitable form isby way of a salt such as the zinc carbonate or zinc phosphate. Likewise,the zinc orthophosphate may be used. Other appropriate zinc compoundsare listed in the above-cited Handbook.

The borate ion in the bonding solution may be supplied by way of anyconvenient source in the form of any soluble borate salts. A preferredform is to supply the borate as boric acid and/or zinc borate. Otherappropriate borate compounds are listed in the Handbook.

The borate, the zinc and the magnesium ions are preferably supplied in aform that is soluble in the phosphoric acid.

The amount of the zinc and/or borate ions to be provided in the bondingsolution should be such as to maintain or bring the pH range in thatspecified of 2 to 4.5.

The order of mixing or addition of the solid components of the bondingsolution is not critical. It is preferred though, that the source of themagnesium ion be added first followed by either the source of the zincand/or borate ions, or the aluminum ions.

In accordance with the invention, the coating slurry composition of theinvention is formed by mixing the above described bonding solution withthe metal particles, preferably the aluminum particles in the form of apowder. It is noteworthy that the bonding solution is essentially inertwith respect to any further reaction with the added aluminum. No visiblereaction between the added aluminum particles and the phosphoric acid isapparent in the coating composition of the invention for at least onehour and the best compositions for a period as long as eight hours.

The bonding solutions of the present invention are particularly usefulfor forming coating compositions for ferrous metal alloy substrates whencombined with particulate aluminum. Particulate aluminums suitable foruse in such coating compositions are well known, and have been discussedat length in the patent literature. For example, such particulatealuminums are set forth in Mosser, U.S. Pat. No. 4,537,632, Mosser etal., U.S. Pat. No. 4,544,408, Mosser et al., U.S. Pat. No. 4,548,646,Mosser et al., U.S. Pat. No. 4,617,056, Mosser et al., U.S. Pat. No.4,659,613, Mosser et al., U.S. Pat. No. 4,863,516, which is particularlydirected to the use of non-leafing aluminum flake in combination withatomized aluminum particles, Mosser, U.S. Pat. No. 4,889,558, Mosser,U.S. Pat. No. 4,537,632 and Mosser et al., U.S. Pat. No. 5,116,672, allof which are incorporated herein by reference. A majority ofchromate/phosphate based compositions that utilize aluminum particlesuse atomized and/or flaked particles of various sizes for coatings withdifferent properties. These are of course also suitable for the presentbonding and coating compositions.

When aluminum is used in the compositions of the invention it may be gasatomized spherical of an average size of 2.5-10 μm ESD, air atomized ofan average size of 4.5-10 μm ESD, flake aluminum; flake/atomizedmixtures; and aluminum alloys. Larger particles as well as smallerparticles can be used.

The coating compositions of the present invention may also include oneor more leachable corrosion inhibitors. Such corrosion inhibitors arediscussed at length in Mosser et al., U.S. Pat. No. 5,116,672,incorporated herein by reference. The leachable pigment is one which iscapable of inhibiting or passivating the corrosion of the metalsubstrate. The leachable pigment is preferably a salt containingenvironmentally acceptable metals like, molybdate, phosphate ormetaborate and combinations, as well as others listed in the publicationby Smith entitled "Inorganic Primer Pigments", Federation Series onCoatings Technology, which is incorporated herein by reference. Goodleachable corrosion inhibitors for use in the coating compositions ofthe present invention are zinc molybdate/phosphate, zinc phosphate andbarium, calcium, zinc borate and zinc aluminum phosphate.

For purposes of the present coating compositions, it is preferable toavoid the use of chromate salts as the leachable pigment, due toenvironmental considerations. However, such chromate salts may be usedas the leachable pigment in applications where such use can betolerated.

The compositions of the invention may, of course, contain othercompatible known ingredients such as surfactants, wetting agents andother conventional additives.

The slurry coating compositions of the invention are applied in aconventional way to the ferrous metal alloy surface to be coated.Manners of application are described in the patents referred to aboveand incorporated herein by reference. Generally, it is desirable todegrease the part to be coated, abrade, roughen and blast with abrasive,apply the coating of the invention by any suitable means, such as byspraying, brushing, dipping, dip spinning, etc., drying until the colorof the coating turns grayish, curing the coating at a temperature ofabout 650° F. (343° C.) for 15 minutes or longer, curing at higher orlower temperatures if desired. The slurry is preferably applied in twocoats, each about 0.00125 inch (0.032 μm) in thickness, then, ifdesired, dried at about 180° F. (82° C.) for 15 to 30 minutes and thencured at 650° F. (340° C.) for 30 to 60 minutes between each coat.Drying is not an essential step.

The coatings as cured at 650° F. (343° C.) are not electricallyconductive and therefore can not provide galvanic protection againstcorrosion of the underlying substrate material. However, the coating maybe made electrically conductive by bead peening or burnishing with glassbeads, abrasive media at low pressure or mechanically cold worked inother ways to produce a conductive sacrificial coating or by heating asspecified in MIL-C-81751B specification (incorporated herein byreference). In this manner the coatings can, by mechanical or thermalprocesses, be made electrically conductive and thereby produce galvanicas well as barrier protection of the underlying ferrous alloy substrate.Desirably, after the second coating is applied, dried, cured andprocessed to make it electrically conductive, the surface of the coatingis sealed with the bonding solution to further increase the oxidationand corrosion protection provided by the coating, and to decrease therate of consumption of aluminum in the coating during service. Thebonding solution can but need not be a bonding solution of theinvention. The seal coats are dried and cured at the same time andtemperature as the above described slurry coatings.

As has been described above, it is an important object of the inventionto provide bonding and coating compositions which are essentially freeof chromate, molybdate and other like toxic or undesirable metals. Insituations where more permissive environmental conditions would permitthe use of such metals as chromium, molybdenum, nickel and others, it isnot excluded that such metals be used in the bonding and/or the coatingcomposition. When it is chromium and/or molybdenum, it would be in anamount of chromium and/or molybdenum which is less than that necessaryto passivate or neutralize the phosphoric acid solution to the reactionwith metallic aluminum. The amount necessary to passivate the bondingsolution as taught in the prior art generally is at least 0.2% by weightof the final coating composition. But by far preferable as disclosedherein are the compositions which are essentially free, if not virtuallyor totally free, of chromate and/or molybdate ions.

The following Examples are merely illustrative of the invention. Theyare not intended as being limiting.

EXAMPLE 1 Bonding Solutions

Test bonding solutions were prepared starting with the following baseformula:

2300 g Deionized water

1326 g Phosphoric acid, 85%

150 g Magnesium oxide

155 g Magnesium carbonate

75 g Aluminum hydroxide

The pH of the bonding solution was 2.0. To the above base formulaadditional components were added to form test solutions. Each testsolution was tested for stability when combined with aluminum powder.

The screening test employed to evaluate the reactivity of a bondingsolution with aluminum metal powder proceeds as follows. Aluminum powder(10-40 grams) were added to 50 Ml of a candidate bonding solution andthe time to noticeable reactivity was measured as well as the violenceof the reaction.

A 100 ml of the above base solution was mixed in the compounds shown inTable 1 below and then mixed with 80 grams of 5 μm aluminum powder andallowed to react. The time until visible reaction was measured (inhours). The test results are presented in Table I:

                  TABLE I                                                         ______________________________________                                              Description                                                                   (changes to 100 mL of)    Hours to                                      Test #                                                                              (Bonding Composition #1)                                                                         pH     Reaction                                                                             Ranking                                ______________________________________                                        1.    5 g    MgCO.sub.3 + 3.5 g ammonium                                                                    2.65                                                                              2      2                                                 molybdate                                                        2.    6 g    ZnCO.sub.3 + 4 g MgCO.sub.3                                                                   2.6  >16    4                                    3.    5 g    MgCO.sub.3 3 + 4 g H.sub.3 BO.sub.3                                                           3.4  >16    4                                    4.    20 g   zinc borate     2.9  6      3                                    5.    20 g   zinc phosphate   3.05                                                                              6      3                                    6.    5 g    ZnCO.sub.3 + 2 g H.sub.3 BO.sub.3                                                              2.95                                                                              >24    4                                    7.    Stetson '488       2.3    0.5    1                                      ______________________________________                                         Rankings 0-1 hour = 1                                                         1.25-4 hours = 2                                                              4.25-8 hours = 3                                                              8+ hours = 4                                                             

EXAMPLE 2

A bonding composition "A" was prepared according to the followingformula:

800 gm Deionized water

388 gm Phosphoric acid, 85%

17.5 gm Zinc oxide

10.3 gm Ferric phosphate

120 gm Magnesium carbonate

31 gm Boric acid

An aluminum coating composition was obtained by admixing the following:

200 ml Bonding composition "A"

50 ml Deionized water

8 gm Zinc aluminum phosphate (Heucophos ZPA)

120 gm Aluminum powder, air atomized, 4.5 μm average particle size

The composition was blended for five minutes and then screened through a325 mesh sieve. This coating was applied to AISI 410 stainless steelpanels in two coats, each cured at 650° F. The coated panels ere testedas described in Table II and compared to panels coated with Example 7 ofAllen '251 and shown to have equivalent performance. Example 7 of Allen'251 meets the requirements of MIL-C-81751B Type 1, Class 4. Thisformulation showed no sign of reactivity of the bonding solution withthe pigment for over 24 hours. The bonding composition "A" contains nodissolved aluminum ion.

EXAMPLE 3

A different bonding composition, "B", was prepared by mixing thefollowing ingredients in the order given:

759 gm Deionized water

398 gm Phosphoric acid, 85%

50 gm Magnesium oxide

51.6 gm Magnesium carbonate

25 gm Aluminum hydroxide, dried gel

The pH of the resulting solution was 2.9.

In order to show that this composition was not "equilibrated" withrespect to aluminum ion and ion source, the batch was split in half andan additional 5 gm of aluminum hydroxide was added (a 40% increase inaluminum hydroxide). The additional aluminum hydroxide dissolved readilygiving a solution with a pH of 3.2. Even though bonding solution "B" wasnot equilibrated with respect to aluminum a stability of 3 hours wasobtained when 40 gm of aluminum powder was added to 50 ml of bondingsolution.

EXAMPLE 4

A bonding solution "C" was prepared according to the following formula:

830 ml Deionized water

287 gm Phosphoric acid, 85%

100 gm Magnesium carbonate

16 gm Aluminum hydroxide, dried gel

8 gm Ferric phosphate

31 gm Boric acid

This composition had a pH of 2.75. It was not "equilibrated" withrespect to any of the dissolved ions. A coating composition was mixedaccording to the following:

100 ml Bonding composition "C"

1 ml Mazon RI 325 Surfactant/corrosion inhibitor

0.1 ml Surfynol 104 surfactant

4.5 gm Flake aluminum powder, Reynolds 4-301

4 gm Zinc phosphate, Heucophos ZPA

66.5 gm Aluminum powder, air atomized, 4.5 μm average particle size

After mixing, the coating showed no sign of binder pigment interactionfor over 8 hours. The coating was spray applied to 410 stainless steelpanels cured at 650F. The panels were scribed with an "X" and placed in5% salt spray per ASTM B117 for 500 hours. After testing there was nosign of corrosion of the substrate, either on the surface or in thescribe. This composition represents the use of flake and particulatealuminum in the same composition.

EXAMPLE 5

The 100 ml of bonding composition "B", three grams of boric acid wasadded. The resulting solution had a pH of 3.0. When 25 ml of deionizedwater, 4 gm of zinc aluminum phosphate, and 60 gm of aluminum metalpowder were added to the solution, a composition which was stable forover 8 hours was obtained. 410 stainless steel panels coated with twocoats of the composition were tested per the 500 hour salt spray testdescribed in Table II. There was no evidence of attack of corrosion ofthe base metal.

EXAMPLE 6

The coating composition of Example 1 was prepared except that inert gasatomized spherical aluminum powder was used in place of air atomizedaluminum. Coated panels were tested per the bend and corrosive oxidationtests described in Table II. There was no coating loss observed on thebend test specimens. Weight change on the corrosive oxidation test wasless than 1 mg/cm². This example shows a useful formulation withspherical aluminum powder instead of air atomized aluminum.

EXAMPLE 7

A coating composition was prepared that contains no carcinogenic,poisonous, heavy metals containing, or otherwise toxic ingredients. Theformula is given as follows:

Bonding Solution

830 gm Deionized water

244 gm Phosphoric Acid

16 gm Aluminum hydroxide, dried gel

8 gm Ferric phosphate

6 gm Zinc carbonate

100 gm Magnesium carbonate

31 gm Boric acid

Coating Composition

100 ml Bonding solution

1 ml Mazon RI325 amine borate corrosion inhibitor

2 Dr Surfynol 104 surfactant

2.5 gm Aluminum flake, Reynolds 4-301

2 gm Zinc phosphate corrosion inhibiting pigment, Nalzin 2

66.5 gm Aluminum powder, Reynolds 1-201 (air atomized aluminum powder,average particle size 4.5 μm)

The coating was applied to representative 403 stainless steel panels intwo separately cured coats (at 343° C. (650° F.)). 410 stainless steelpanels were also coated with SermeTel W, an aluminum filled ceramiccoating containing chromium made in accordance with Allen '251. Bothcoatings were burnished with alumina grit to achieve a sacrificialcoating. Table II gives a list of tests performed and their results withthe No Cr coating and Allen '251, Example 7.

                  TABLE II                                                        ______________________________________                                        PERFORMANCE DATA                                                                                           Ex-     Allen                                                                 ample   '251                                                                  Nos.    Exam.                                    TEST DESCRIPTION                                                                          REQUIREMENT      1-7, 9  No. 7                                    ______________________________________                                        1000 hrs. salt spray                                                                      no galvanic attack of base                                                                     Pass    Pass                                     ASTM B117   metal.sup.1                                                       500 hrs. Salt Spray                                                                       no base metal attach (white                                                                    Pass    Pass                                     per ASTM B117                                                                             corrosion products present)                                       scribed "X"                                                                   Bend test, 90° around                                                              no separation from base                                                                        Pass    Pass                                     14X mandrel metal.sup.1                                                       Abrasion resistance,                                                                      >100 L/mil.sup.1 Pass    Pass                                     Falling sand                                                                  per ASTM D968                                                                 Oxidation resistance,                                                                     <1 mg/cm.sup.2 weight change                                                                   Pass    Pass                                     100 hrs at                                                                    1000° G. (538° C.)                                              Corrosive oxidation,                                                                      no cracking, pitting,                                                                          Pass    Pass                                     24 hrs. salt spray                                                                        blistering <1 mg/cm.sub.2 weight                                  then 100 hrs @                                                                            change                                                            1000° F.                                                               Thermal salt spray,                                                                       no cracking, pitting,                                                                          Pass    Pass                                     100 hrs @ 1000° F.                                                                 blistering <5 mg/cm.sub.2 weight                                  then 1000 hrs                                                                             change                                                            salt spray                                                                    Hot Water Resistance,                                                                     no cracking, blistering, pass                                                                  Pass    Pass                                     boiling water 10 min                                                                      bend test                                                         Fuel Resistance,                                                                          pass bend test after                                                                           Pass    Pass                                     4 hrs immersion in                                                                        immersion                                                         ASTM Ref Fuel B                                                               Hot Oil Resistance,                                                                       no peeling, blistering,                                                                        Pass    Pass                                     8 hrs immersion in                                                                        softening                                                         di-2-ethyl-                                                                   hexyl sebacate + 1/2%                                                         phenothiazine at                                                              400° F.                                                                Hydraulic Fluid                                                                           no blistering, wrinkling,                                                                      Pass    Pass                                     Resistance, 24 hrs                                                                        softening                                                         immersion in                                                                  Skydrol at 300° F.                                                     ______________________________________                                         .sup.1 Test and requirements per MilC-81751 Type 1 MetallicCeramic            Coating.                                                                 

Comparative Example 8

A comparative example, not in accordance with the present invention wasprepared, using the composition set forth below, which is factored tocompare with the base formula used in Example 1:

2300 g Deionized water

1018 g Phosphoric acid, 85%

354 g Magnesium carbonate

74 g Aluminum hydroxide

The stability test as described in Example 1 was carried out on thiscomposition, and a time to reactivity of 0.5 hours was measured. Thisexample falls within the preferred range of compositions of Stetson'488. It has low stability.

EXAMPLE 9 Coating Composition

A sample composition was prepared by making the following bondingsolution and adding aluminum powder in the ratio given:

Bonding Solution

850 g Deionized water

287 g Phosphoric acid, 85%

100 g Magnesium carbonate

10 g Aluminum hydroxide

9 g Zinc carbonate

20 g Boric acid

Coating Composition

150 mL Bonding solution

80 g Aluminum powder

4 g Zinc molybdate/phosphate

0.25 mL Surfactant (sorbitan monostearate)

Coated mild steel test panels were prepared and placed in salt spray.There were decreased amounts of sacrificial corrosion on the surfacewhen compared with a similar formulation without the leachable corrosioninhibitor (zinc molybdate/phosphate).

In addition, another test was conducted using the same compositionexcept that 4 g of a non-leafing flake aluminum was added. Thiscomposition showed improved corrosion resistance over the compositionwithout the flake aluminum. The use of flake aluminum in combinationwith particulate aluminum is discussed in Mosser et al., U.S. Pat. No.4,863,516, cited above.

The coating compositions of the invention can be applied to any ferrousmetal alloy surfaces including cast iron, mild steel, low alloy steels,the 300 series stainless steels, nickel based alloys and titaniumalloys. The coating compositions are designed particularly for turbinecompressor air foils, such as blade, vanes, stators, and other parts.Such components are usually made of the alloys of the following: 410Stainless Steel (AMS5504); 403 Stainless Steel; AM355 (15.5 Cr, 4.5 Ni,2.9 Mo, 0.85 Mn, 0.12 C 0.09 n balance Fe); AM350 (16.5 Cr, 4.5 Ni, 2.9Mo, 0.85 Mn, 0.10 C 0.10N balance Fe); 430 Stainless (17.0% Cr); and17-4 pH (16.1-16.5 Cr, 4.0-4.1 Ni, 0.28-0.3 Ta/Cb, 3.1-4.0 Cu, balanceFe).

The invention also covers the parts coated with the coating compositionof the invention.

It will be understood that various changes in the details, materials andarrangement of parts which have been herein described and illustrated inorder to explain the nature of this invention may be made by thoseskilled in the art within the principle and scope of the invention asexpressed in the following claims.

What is claimed is:
 1. An aqueous phosphoric acid bonding solution whichcomprises a source of magnesium ions, a source of aluminum ions, and asource of zinc ions, the solution having a pH in the range of 2 to 4.5,and the amount of aluminum in solution being less than necessary toequilibrate the solution with respect to addition of aluminum, thesolution being substantially free of chromate and molybdate.
 2. Theaqueous phosphoric acid bonding solution of claim 1 wherein the sourceof the aluminum ions is aluminum hydroxide.
 3. The bonding solution ofclaim 1 which is substantially free of vanadium.
 4. The aqueousphosphoric acid bonding solution of claim 1, which contains a source ofborate ions.
 5. The aqueous phosphoric acid bonding solution of claim 4,wherein the source of magnesium ions is magnesium oxide and/or magnesiumcarbonate, the source of zinc ions is zinc oxide, zinc carbonate, zinchydroxide, zinc phosphate, or zinc borate and the source of borate ionsis boric acid.
 6. The aqueous phosphoric acid bonding solution of claim5 which comprises aluminum hydroxide as the source of aluminum ions. 7.The aqueous phosphoric acid bonding solution of claim 4 which comprisesaluminum hydroxide as the source of aluminum ions.
 8. An aqueousphosphoric acid bonding solution which comprises a source of magnesiumions and a source of aluminum ions, the solution having a pH in therange of 2 to 4.5, and the amount of aluminum in solution being lessthan necessary to equilibrate the solution with respect to addition ofaluminum particles, and which bonding solution contains a source of zincand borate ions.
 9. The aqueous phosphoric acid bonding solution ofclaim 8, wherein the source of magnesium ions is magnesium oxide and/ormagnesium carbonate, the source of zinc ions is zinc oxide, zinccarbonate, zinc hydroxide, zinc phosphate, zinc borate or zincmolybdate/phosphate and the source of borate ions is boric acid.
 10. Thebonding solution of claim 8 which is substantially free of vanadium. 11.The aqueous phosphoric acid bonding solution of claim 8 wherein thesource of aluminum ions is aluminum hydroxide.